The present invention relates to the manufacture of curved glass articles, and more particularly to the manufacture of lenses for sunglasses by sag-forming the same from a sheet of flat glass.
In current operation, individual pieces of glass, usually cut from a flat sheet, are sagged from their original form by placing them upon a curved mold so as to be supported on their periphery, and then subjecting them to a temperature cycle which softens the glass for sagging, and then the glass is annealed. At the time the glass is softened, it may be sagged by natural gravity flow, or by additional pressing, with or without contact with the curved sidewall of the mold. It is advisable to shorten the softening cycle when there is contact with the sidewall, so as to preserve the original surface quality of the glass. In general, should the glass, while at high temperatures, contact another surface, it is usually necessary to polish the glass to restore its optical qualities, especially with glasses for use in the spectacle-making industry.
The normal process of manufacturing lenses for sun-glasses includes the sagging of a plurality of individual round, square and other forms of "coquilles" from flat glass. The lenses are then cut out from the sagged round or square coquilles and may undergo diverse finishing operations. The coquilles are considered to be semifinished products which should possess certain properties of the final product, notably the properties of transmission, color and optical quality, where optical quality includes glass quality as well as physically measured optical parameters.
The essential properties of the lenses or of the sagged blank, are its radius of curvature, thickness and index of refraction. Using an approximation from thick lens formulae shows that the transmitted power of a sagged lens (concentric radii of curvature) is given by ##EQU1## where P is the optical power, d the lens thickness, .eta. the refractive index, and R.sup.2 .apprxeq.R.sub.1 R.sub.2, the radii of curvature. That is, the optical quality of a coquille is very sensitive to its curvature; and accordingly it is the aim of all sagging processes to obtain a regular curvature for a single lens which is consistent from one to another.
The power P of a curved surface is expressed in diopters by the following relation: ##EQU2## .eta. BEING THE REFRACTIVE INDEX OF THE LIGHT. Thus both transmitted power and surface power are sensitive to the radius of curvature. The regularity of the curvature depends upon the sagging quality. In the manufacture of lenses for safety spectacles, the sagging is effected without the central part of the lens coming into contact with the mold. The best known sagging is obtained by maintaining the periphery of the glass piece to be sagged in a fixed position, such as by clamping; whereas a lesser but nevertheless acceptable quality may be obtained by simply placing the periphery of the piece in contact with the mold.
In both of these known methods, the periphery of the blanks presents a different curvature to that of the center of the sagged piece, and also a surface state which is incompatible with good optical quality. Accordingly, it is necessary to cut the required lens contour from a central portion of the sagged piece in order to avoid the nonuniformly sagged peripheral portion. Thus, in the known processes, the obtainment of a good lens necessitates two cutting operations: firstly, the cutting out of an overdimensioned disc in flat glass for the sagging operation, and then the cutting out of the final form from the center of the sagged piece.
A further known method of forming said pieces is to initially cut out the defined size and shape from flat glass. However the sagging is then more delicate, as it is difficult to assure limited contact between the mold and the sagged form. The regularity of curvature is therefore, as a general rule, less good than that which would be obtained with the above-described process using two cutting operations.
Accordingly, the present invention permits the obtainment of a good regularity to curvature while avoiding a double cutting operation, and further reduces glass losses.